Differentiated Services Y.Bernet, et al
Internet Draft February, 1999
Document: draft-ietf-diffserv-framework-02.txt
Yoram Bernet, Microsoft
James Binder, 3-Com
Steven Blake, Torrent Networking Technologies
Mark Carlson, Redcape Software
Brian E. Carpenter, IBM
Srinivasan Keshav, Cornell University
Elwyn Davies, Nortel Networks
Borje Ohlman, Ericsson
Dinesh Verma, IBM
Zheng Wang, Bell Labs Lucent Technologies
Walter Weiss, Lucent Technologies
A Framework for Differentiated Services
Status of this Memo
This document is an Internet Draft and is in full conformance with
all provisions of Section 10 of RFC2026. Internet Drafts are
working documents of the Internet Engineering Task Force (IETF),
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Community. Discussion and suggestions for improvement are
requested. This document will expire before September, 1999.
Distribution of this draft is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1999). All Rights Reserved.
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CONTENTS
1. Abstract......................................................4
2. Structure of this Draft.......................................4
3. Differentiated Services - Motivation and Definition...........4
4. Services......................................................5
4.1 Customer/Provider Boundaries..............................5
4.2 SLSs and TCSs.............................................6
4.3 Service Taxonomy: Quantitative through Qualitative and
alternatives..........................................7
4.4 The Scope of a Service....................................8
4.4.1 Services where the Scope is Tied to the Receiver....8
4.5 Dynamic vs. Static SLSs...................................9
4.6 Provisioning Traffic Conditioners in Boundary Devices to
Provide Services.....................................10
4.6.1 Minimal Functionality at Provider's Ingress........11
4.6.2 Functionality at Provider's Ingress for Double-ended
SLSs.............................................12
4.6.3 Added Value Functionality at Provider's Ingress....12
4.6.4 Functionality at Customer's Egress.................13
4.6.5 Functionality at Provider's Egress.................13
4.7 Internal Provisioning....................................14
4.8 End-to-End Service Construction..........................14
5. Service Examples.............................................14
5.1 Better than Best-Effort (BBE) Service....................14
5.1.1 Service Implementation.............................15
5.2 Leased Line Emulation Service............................16
5.2.1 Service Implementation.............................16
5.3 Quantitative Assured Media Playback Service..............17
5.3.1 Service Implementation.............................17
5.4 Superposition of Quantitative and Qualitative Services in
the Same Network.....................................18
6. Provisioning and Configuration...............................18
6.1 Boundary vs. Interior Provisioning and Configuration.....19
6.1.1 Boundary Provisioning..............................19
6.1.2 Interior Provisioning..............................20
6.1.2.1 Quantitative Provisioning of the Interior..21
6.1.2.2 Qualitative Provisioning of the Interior...22
6.2. Static vs. Dynamic Provisioning.........................23
6.3 Distributing Configuration Information...................24
6.3.1 Top Down Distribution of Configuration Information.24
6.3.2 Distribution of Configuration Information via
Signaling........................................25
6.3.3 Modification of Configuration Information Based on
Real-Time Measurement............................26
7. Inter-Domain Considerations and End-to-End Services..........26
7.1 TCSs.....................................................27
7.2 Inter-Domain Provisioning................................27
7.3 Service, PHB and Codepoint Mapping.......................28
7.4 Host-Domain Boundaries...................................29
8. Deployment Scenarios.........................................29
9. Inter-operability with RSVP/Integrated Services..............31
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9.1 RSVP/Integrated Services Over Differentiated Services....31
9.2 Parallel Operation.......................................32
10. Multicast Services..........................................32
10.1 Codepoints and PHBs for Multicast Services.............33
10.2 Provisioning Multicast Services........................33
11. Security and Tunneling Considerations.......................34
12. Acknowledgements............................................35
13. References..................................................35
14. Author's Addresses..........................................36
Changes from previous version
- Terminology made consistent with architecture _ particularly
boundary (node) used in place of edge (node) where appropriate.
- Table of contents added.
- Traffic Conditioning Agreement (TCA) replaced by Traffic
Conditioning Specification (TCS) throughout to avoid
connotations of contractual agreement.
- Most instances of Service Level Agreement (SLA) replaced by
Service Level Specification (SLS) where it is clear that we are
talking about the technical specification of the services. The
SLS is defined as the technical specification part of the
contractual SLA. Emphasized that this document discusses the
technical aspects of the SLA whilst acknowledging that it fits
in a wider contractual framework which is outside the scope of
technical standards.
- Deployment scenarios section added.
- Whole document coordinated with [DiffEdge] and [E2EQoS].
- Service scope added as a component of TCS.
- Connections made to work of MPLS and IP Performance Metrics
WGs.
- Pointed out that dealing with the interactions of multiple end-
to-end services is an open question and is unlikely to have a
computable answer in common cases.
- Multicast section improved:
- Added preamble pointing out that DS should be good for
multicast except that provisioning is difficult
- Application level unicast is dealt with by multiple
instances of point-to-point services
- Pointed out that provisioning multiple source mpt-to-mpt is
not a straight generalisation of pt-to-mpt
- Emphasised that TC for a quantitative service in an IPsec
tunnel will be difficult to realize because the relevant packet
fields are hidden.
- Updated references to reflect current drafts. Added a few new
references including [ROTZY] for bandwidth broker.
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1. Abstract
This document provides a general description of issues related to
the definition, configuration and management of services enabled
by the differentiated services architecture [DSARCH]. This
document should be read along with its companion documents, the
differentiated services architecture [DSARCH] and the definition
of the DS field [DSHEAD]. A glossary of specialist terms used may
be found in [DSARCH].
2. Structure of this Draft
Section 3 defines Differentiated Services and explains the
motivation behind its deployment. Section 4 defines the concept of
a service and the components that comprise a service. Section 5
discusses several service examples. Section 6 examines intra-
domain provisioning, configuration and management issues. Section
7 examines inter-domain provisioning, configuration and
management. Section 8 describes some possible deployment
scenarios. Section 9 addresses interoperability with Integrated
Services and RSVP. Section 10 discusses the interaction of
differentiated services with multicast and tunneling. Section 11
addresses security concerns.
3. Differentiated Services - Motivation and Definition
Traditionally, network service providers (both enterprise and
traditional ISPs) provide all customers with the same level of
performance (best-effort service). Most service differentiation
has been in the pricing structure (individual vs. business rates)
or the connectivity type (dial-up access vs. leased line, etc.).
However, in recent years, increased usage of the Internet has
resulted in scarcity of network capacity, compromising performance
of traditional, mission critical applications. At the same time,
new applications have emerged which demand much improved service
quality. As a result, service providers are finding it necessary
to offer their customers alternative levels of service. As well as
meeting new customer expectations, this allows service providers
to improve their revenues through premium pricing and competitive
differentiation of service offerings, which in turn can fund the
necessary expansion of the network.
The Differentiated Services architecture offers a framework within
which service providers can offer each customer a range of network
services which are differentiated on the basis of performance in
addition to pricing tiers used in the past. Customers request a
specific performance level on a packet by packet basis, by marking
the DS field of each packet with a specific value (see [DSHEAD]
for more details). This value specifies the Per-hop Behavior (PHB)
to be allotted to the packet within the provider's network.
Typically, the customer and provider negotiate a profile (policing
profile) describing the rate at which traffic can be submitted at
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each service level. Packets submitted in excess of this profile
may not be allotted the service level requested. A salient feature
of differentiated services is its scalability, which allows it to
be deployed in very large networks. This scalability is achieved
by forcing as much complexity out of the core of the network into
boundary devices which process lower volumes of traffic and lesser
numbers of flows, and offering services for aggregated traffic
rather than on a per-micro-flow basis.
4. Services
[DSARCH] defines a Service as "the overall treatment of a defined
subset of a customer's traffic within a DS-domain, or end-to-end".
Although PHBs are at the heart of the differentiated services
architecture, it is the service obtained as a result of marking
traffic for a specific PHB, which is of value to the customer.
PHBs are merely building blocks for services. Service providers
combine PHB implementations with traffic conditioners,
provisioning strategies and billing models which enable them to
offer services to their customers. Providers and customers
negotiate agreements with respect to the services to be provided
at each customer/provider boundary. These are commonly referred to
as Service Level Agreements (SLAs). Many of the aspects of SLAs
(such as payment terms) are beyond the scope of technical
standards and are therefore not considered in this document; the
subset of the SLA which provides the technical specification of
the service will be referred to as the Service Level Specification
(SLS).
Bear in mind when considering the services that are offered in a
DS-domain that:
* DS services are all for unidirectional traffic only
* DS services are for traffic aggregates, not individual micro-
flows
4.1 Customer/Provider Boundaries
Present day network traffic generally traverses a concatenation of
networks which may include hosts, home or office networks, campus
or corporate networks and several large transit networks. Home and
office networks are typically customers of campus or corporate
networks, which are in turn customers of large transit networks.
While one would expect the initial deployment of differentiated
services to be within large transit networks, its deployment may
also be extended to the smaller campus and corporate networks and
in special cases, all the way to individual hosts. As such, there
may be numerous customer/provider boundaries at which the concept
of a 'service' applies.
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4.2 SLSs and TCSs
At each differentiated service customer/provider boundary, the
technical aspects of the service provided is defined in the form
of an SLS which specifies the overall features and performance
which can be expected by the customer. Because DS services are
unidirectional the two directions of flow across the boundary will
need to be considered separately. An important subset of the SLS
is the traffic conditioning specification, or TCS. The TCS
specifies detailed service parameters for each service level. Such
parameters include:
1. Detailed service performance parameters such as expected
throughput, drop probability, latency.
2. Constraints on the ingress and egress points at which the
service is provided, indicating the `scope' of the service.
Service scopes are discussed further in Sec. 4.4.
3. Traffic profiles which must be adhered to for the requested
service to be provided, such as token bucket parameters.
4. Disposition of traffic submitted in excess of the specified
profile.
5. Marking services provided.
6. Shaping services provided.
The TCS, the logical components needed to implement it and the
configuration needed for those components are discussed in more
detail in [DiffEdge].
In addition to the details in the TCS, the SLS may specify more
general service characteristics such as:
1. Availability/Reliability, which may include behavior in the
event of failures resulting in rerouting of traffic
2. Encryption services
3. Routing constraints
4. Authentication mechanisms
5. Mechanisms for monitoring and auditing the service
6. Responsibilities such as location of the equipment and
functionality, action if the contract is broken, support
capabilities
7. Pricing and billing mechanisms
These additional characteristics are important, and in the case of
pricing and billing, fundamental to the service offering but they
do not affect the service itself and are not considered further
here.
Metrics which can be used to validate the delivery of the service
specified by a TCS have been studied by the IP Performance Metrics
Working Group of the IETF and are being published as Informational
RFCs.
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4.3 Service Taxonomy: Quantitative through Qualitative and
alternatives
The Differentiated Services architecture can support a broad
spectrum of different kinds of service. Categorizing these
services provides some constraints on the corresponding SLSs that
can be offered for the service.
Some services can be clearly categorized as qualitative or
quantitative depending on the type of performance parameters
offered. Examples of qualitative services are as follows:
1. Traffic offered at service level A will be delivered with low
latency.
2. Traffic offered at service level B will be delivered with low
loss.
The assurances offered in examples 1 and 2 are relative and can
only be verified by comparison.
Examples of quantitative services are as follows:
3. 90% of in profile traffic delivered at service level C will
experience no more than 50 msec latency.
4. 95% of in profile traffic delivered at service level D will be
delivered.
Examples 3 and 4 both provide concrete guarantees that could be
verified by suitable measurements on the example service
irrespective of any other services offered in parallel with it.
There are also services which are not readily categorized as
qualitative or quantitative as in the following examples:
5. Traffic offered at service level E will be allotted twice the
bandwidth of traffic delivered at service level F.
6. Traffic with drop precedence AF12 has a higher probability of
delivery than traffic with drop precedence AF13 [AF].
In example 5, the provider is quantifying the relative benefit of
submitting traffic at service level E vs. service level F, but the
customer cannot expect any particular quantifiable throughput.
This can be described as a `Relative Quantification Service'.
In general, when a provider offers a quantitative service, it will
be necessary to specify quantitative policing profiles. In many
cases, quantitative policing profiles will be specified even for
services that do not offer quantitative performance.
Determining how to monitor and audit the delivery of a qualitative
or relative quantification service in such a way as to convince
the user that he has received fair measure requires careful
attention. It will be up to the customer to determine if the
advantage offered is sufficient to make the service worthwhile.
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The SLS must clearly avoid making quantitative commitments for
these services.
4.4 The Scope of a Service
The scope of a service refers to the topological extent over which
the service is offered. For example, assume that a provider offers
a service to a customer which connects to their network at ingress
point A. The service may apply to:
1. all traffic from ingress point A to any egress point
2. all traffic between ingress point A and egress point B
3. all traffic from ingress point A to a set of egress points
Egress points may be in the same DS Domain as the ingress point or
may be in other domains which are either directly or indirectly
connected to the ingress domain. If the egress point is in another
domain, it will be necessary for the ingress provider to negotiate
SLAs with the relevant peer domains, which will recursively
negotiate with their peers to ensure that the service offered at
ingress point A can indeed be extended to the egress points
specified. The scope of a service is part of the TCS governing
ingress point A.
In general, providers will be able to offer quantitative services
most efficiently when a specific set of egress points is
specified. Quantitative services which span multiple domains also
require tighter coupling between the SLA offered to the customer
at ingress point A and the SLAs negotiated with intermediate
domains. Qualitative services can more readily be offered to
arbitrary sets of egress points and require looser coupling
between the SLA at ingress point A and SLAs at intermediate domain
boundaries.
4.4.1 Services where the Scope is Tied to the Receiver
A special case of service scope is a service that governs all
traffic between any ingress point and egress point B. The SLS that
defines this service would be at egress point B and would
effectively allow the customer to control the mix of traffic
received from the provider. While such a service is theoretically
possible, it appears to be a mismatch with the more usual
specification of Differentiated Services which governs the
quality with which traffic is sent, rather than received.
A number of concerns would have to be addressed by such a service,
including:
- Traffic going to point B from an ingress point A under the
terms of the SLS of this service may also be governed by an SLS
for traffic submitted at point A. The SLSs may conflict and it
will not, in general, be possible to resolve all such conflicts
across all the ingress points
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- Establishing a traffic profile for this service at every
possible ingress which prevents overload of the receiver can be
more complex than for other service scopes: Static profiles are
likely to be either inefficient (e.g. dividing the egress
profile into fixed proportions) or risky (e.g. allowing every
ingress to send the whole profile) whilst dynamic profiles
require processes and communication mechanisms to coordinate
the settings. For example, the sources may offer 1Mb/s when
the receiver can only accept 9.6Kb/s.
- Without effective ingress profiles for the service, denial of
service attacks will be a serious problem.
Some of the characteristics of receiver oriented services can be
provided by local policies and the SLS for the domain to which
traffic is sent via the egress point as described in Sec. 4.6.4.
4.5 Dynamic vs. Static SLSs
SLSs may be static or dynamic. Static SLSs are the norm at the
present time. These are instantiated as a result of negotiation
between human agents representing provider and customer. A static
SLS is first instantiated at the agreed upon service start date
and may periodically be renegotiated (on the order of days or
weeks or months). The SLS may specify that service levels change
at certain times of day or certain days of the week, but the
agreement itself remains static.
Dynamic SLSs, on the other hand, may change frequently. Such
changes may result for example, from variations in offered traffic
load relative to preset thresholds or from changes in pricing
offered by the provider as the traffic load fluctuates. Dynamic
SLSs change without human intervention and thus require an
automated agent and protocol, for example, a bandwidth broker to
represent the differentiated service provider's domain (such as
suggested in [ROTZY]).
Dynamic SLSs also present challenging problems to both end users
and network providers:
- Network providers have to balance frequently changing loads on
different routes within the provider network. This requires the
provider to adopt dynamic, automated resource provisioning
mechanisms rather than relying on static provisioning.
- Customer equipment will have to adapt to dynamic SLSs in order
to make the most out of the changing SLS.
- End user applications may have to adapt their behavior during a
session to make the most of (or even, cope with) dynamic SLSs.
It is worth reiterating that the SLSs in Differentiated Services
apply to aggregates of traffic and not individual flows. For
scalability, it is undesirable to envisage modifying an SLS every
time a new micro-flow is added or removed from an aggregate.
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4.6 Provisioning Traffic Conditioners in Boundary Devices to Provide
Services
Once an SLS has been negotiated, the service provider (and
optionally the customer) will configure traffic conditioning
components at the boundary between the two networks. The service
provider does so with the goal of protecting the provider's
network such that the resources granted to the customer meet but
do not exceed the terms of the TCS. The customer does so with the
goal of making the best use of the service purchased from the
provider. In this section, we will briefly describe configuration
of traffic conditioners in boundary devices. Traffic conditioners
and their configuration are described in greater detail in
[DiffEdge].
Note that the provider's self interests require only that the
provider identify
- for which service level specific traffic is submitted,
- by which customer it is submitted, and
- for traffic with double-ended SLSs (i.e. SLS scope is type 2 or
3 of Sec. 4.4) only, the destination address to which the
traffic is directed.
Customer traffic may be authenticated either by the physical
connection on which it arrives or by some sophisticated
cryptographic means which is beyond the scope of this draft. The
provider need not be concerned with the customer's individual
micro-flows in delivering basic Differentiated Services (see Sec.
4.6.3 for additional services).
[DSARCH] identifies four traffic conditioning components:
1. Meters
2. Markers
3. Shapers
4. Droppers
The combination and interaction of the traffic conditioning
components is selected on a packet-by-packet basis by the DS
codepoint. The configuration parameters for the components at
each codepoint are determined by the policies and profiles
applied, so that the conditioner polices the traffic in the BA
specified by the codepoint. Meters measure submitted traffic for
conformance to a profile, providing control input for the other
components which implement the policing:
- Shapers police by delaying submitted traffic such that it does
not exceed the traffic rate specified in a profile.
- Droppers police by dropping traffic that is submitted at a rate
exceeding that specified in a profile.
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- Markers police by re-marking the traffic with a different
codepoint either
- to demote out-of-profile traffic to a different PHB,
- as a result of an SLS which specifies codepoint mutation, or
- to ensure that only valid codepoints are used within the
domain.
In addition to these four components, traffic classifiers are
required in order to separate submitted traffic into different
classes. Classifiers may separate traffic based only on the DS-
field of submitted packets (BA classifiers) or may do so based on
multiple fields within the packet header and even the packet
payload (MF classifiers). MF classifiers may be used at
boundaries to provide certain per-micro-flow services to
customers. Examples of such services include per-flow marking or
shaping. Typically, traffic will arrive at the boundary of a DS
domain pre-marked and pre-shaped. However, at interfaces with some
non-DS customer networks, it is possible that traffic will require
marking and shaping.
Even if a customer has pre-marked and pre-shaped, the service
provider will wish to police the traffic at the ingress boundary
to meet the domain's self-interests. This may result in traffic
being re-marked or dropped.
Traffic conditioning components (in particular, meters) will also
be the primary source of accounting information for a
Differentiated Services network.
4.6.1 Minimal Functionality at Provider's Ingress
At the very least, the service provider must limit traffic carried
on behalf of the customer to the constraints specified in the TCS.
A simplified TCS can be represented in the form of a table wherein
each row has the format:
DS-Mark : Profile : Scope : Disposition of non-conforming traffic
This row indicates that the provider commits to carry traffic
marked with 'DS-Mark' at the corresponding service level, provided
that it conforms to the 'Profile'. Traffic that is submitted with
'DS-Mark' and which does not conform to the 'Profile', is
subjected to 'Disposition of non-conforming traffic'. This is
generally a policing action such as re-marking to a lower service
level, delaying in a shaper, or dropping. Alternatively, it may be
carried at the requested service level, but subjected to a
surcharge. The scope for this type of service would normally be
expected to be of type 1 of Sec. 4.4.1, where the traffic
destination can take it through any egress point of the domain.
To provide this minimal functionality, the provider must configure
a BA classifier to separate traffic into the different service
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level requested, based on DS-Mark. Following the BA classifier,
each class must be metered for conformance to the corresponding
profile. Following the profiler, either a dropper, shaper or re-
marker is likely to be employed. The Better than Best Efforts
service described in Sec. 5.1 is an example of a service for which
this functionality is sufficient.
4.6.2 Functionality at Provider's Ingress for Double-ended SLSs
If quantitative or other services needing double-ended SLSs (types
2 and 3 of Sec. 4.4.1) are implemented in a DS Domain, these
services specify the possible egress port(s) for traffic
conforming to the SLS. The traffic conditioner needs to consider
the destination address of the packet as additional input to the
policing process, so that traffic is not accepted for egress ports
for which an SLS does not exist. The Virtual Leased Line service
described in Sec. 5.2 is an example of a service that would
require this functionality.
A QoS VPN can be constructed by provisioning multiple instances of
services of type 2, building in effect, a mesh of point to point
QoS links.
Services of type 3 are most likely to be used for multicast
applications (see Sec. 10).
4.6.3 Added Value Functionality at Provider's Ingress
The functionality described in Secs. 4.6.1 and 4.6.2 serves only
to protect the provider's network resources in line with the terms
of the TCS. It provides no assistance to the customer. The burden
of marking packets and shaping traffic falls entirely on the
customer. In some cases, the SLS may call for the provider to
provide additional services to the customer. Such services may
include:
1. Marking traffic from specific micro-flows to a specific
behaviour aggregate (marking the DS-field).
2. Policing traffic from specific micro-flows or sets of micro-
flows, either in the form of dropping or shaping.
In order to provide such services, the provider must generally
employ an MF classifier in addition to the BA classifier. The need
for an MF classifier arises only when the customer requires the
provider to provide some form of traffic separation or
authentication on behalf of the customer. The provider may charge
dearly for these services depending on the degree of granularity
and the amount of work required. For example, shaping thousands of
customer micro-flows might consume considerable resources in the
provider's boundary device. On the other hand marking based on
source subnet addresses would consume considerably fewer
resources.
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4.6.4 Functionality at Customer's Egress
Strictly speaking, the customer need not apply any specific
traffic conditioning. In this case, the customer relies on the
provider to mark as per negotiated MF classification criteria. In
many cases it is preferable for the customer to mark. Customer
marking may be necessary when customer packets are encrypted (as
in the case of end-to-end IPSec). Customer marking enables the
customer to direct specific traffic from specific users or
applications to specific service classes. This may be difficult or
impossible for a provider to do on behalf of a customer when, for
example, applications use volatile ports and users are assigned
IP addresses based on DHCP.
In addition to marking, it is in the customer's best interest to
at least shape per service level, at the customer network's egress
point. Otherwise, customer traffic may be policed by the service
provider with undesirable consequences (e.g. dropped packets).
Shaping per service level does not however, provide for micro-flow
traffic separation. As a consequence, a renegade traffic source
may cause the profile to be exceeded for a specific service level,
negatively impacting all customer flows which are marked for that
service level. Therefore, it is often in the customer's interest
to meter and shape or at least to police, with micro-flow
granularity.
4.6.5 Functionality at Provider's Egress
At the egress from a provider's domain there may be an SLA in
place with a peer DS domain, which might be either another
provider or an end user domain. As in Sec. 4.6.4, it is in the
provider's best interests to shape the traffic leaving the domain.
Depending on the SLA, the egress may be required to remark and/or
police or shape the traffic. Note that the forwarding treatment
applied to the packet in the egress node of the domain would be
that selected by the codepoint before it was remarked (otherwise,
the egress node has to support multiple codepoint to PHB
mappings).
If the peer domain is a non-DS domain the egress may be required
to remark all packets to conform to one of the previous standards
for the use of the TOS byte [RFC791,RFC1349].
The provider may also wish to offer additional services to a
customer by policing egress traffic with micro-flow granularity if
the customer might expect to receive excessive traffic in a single
BA and wishes to apply greater control than could be achieved by
normal policing of the aggregate. This would be specified via an
SLS in the usual way.
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4.7 Internal Provisioning
The provider must provision internal nodes in the provider network
to meet the assurances offered by SLSs negotiated at the
boundaries of the network. To do so, the provider may use similar
traffic conditioning mechanisms to those used at the network
boundaries. However, providers are unlikely to apply MF
classification within the interior of the network. The provider
may police periodically within the network, by reshaping,
remarking or discarding traffic.
Service providers are experienced in provisioning large networks
which offer uniform service. They are assisted in this by
predictive tools, traffic modeling tools and real time
measurements. Current techniques will likely be applied to
differentiated services networks, although, the complexity of
provisioning will increase significantly. In a differentiated
service network, the provider must ensure that resources granted
to traffic of one service level does not inappropriately
compromise assurances regarding traffic at other service levels
(for example, in example service 6, traffic in AF12 can
legitimately compromise traffic in AF13 if an increase in AF12
traffic causes more AF13 traffic to be dropped). As mentioned
previously, internal provisioning in the case of dynamic SLSs will
likely require dynamic resource allocation protocols.
4.8 End-to-End Service Construction
The Differentiated Services architecture proposes that an end-to-
end service can be constructed by the concatenation of domain
services and their associated customer-provider SLAs for each of
the domains which the service traffic has to cross.
Clearly, not all PHBs and services can be meaningfully
concatenated, and the definition of suitable services and their
associated PHBs will be a major focus of future Differentiated
Services development. This is discussed at greater length in Sec.
7.
5. Service Examples
In this section, we describe service examples and show how they
can be supported by specific PHBs. We base these examples on PHBs
which are defined in [AF]and [EF]. These examples are intended to
be illustrative of the wide range of services that can be employed
using the differentiated services model, and are not intended to
be an exhaustive list. Further examples can be found in the
Appendix of [AF] (`Olympic' service _ related gold, silver and
bronze service levels, a proportional bandwidth service and an
alternative for a low latency service) and [2BIT].
5.1 Better than Best-Effort (BBE) Service
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This is a qualitative service which promises to carry specific web
server traffic at a higher priority than competing best-effort
traffic. Such a service offers relatively loose (not quantifiable)
performance from a given ingress point to any egress point. Such a
service is suitable for example for businesses offering access to
web based content. The BBE service enables the web content
provider to provide content at a generally higher rate than other
content providers are able to, in so reducing the latency
experienced by consumers of the web site.
5.1.1 Service Implementation
In this example, we assume that there is an SLS which defines the
service at the customer's ingress point. This is the point at
which the customer injects web server responses into the
differentiated services network. The information in the TCS can be
represented in the following form [AF]:
AF11 Mark : 1 Mbps : Any egress point : Excess traffic handled by
marking with AF13 mark : .
Packets submitted for the BBE service should be marked with the
DS- field codepoint corresponding to the AF11 PHB. The provider is
promising to carry up to 1 Mbps of traffic from the ingress point
to any egress point at a higher priority than best-effort traffic.
A lesser class of service corresponding to the AF13 PHB will be
applied to traffic submitted for the AF11 PHB, in excess of 1
Mbps.
The provider will provision a policer at the ingress point.
Traffic submitted up to the 1 Mbps limit will be directed to the
AF11 PHB. Traffic submitted in excess of 1 Mbps will be remarked
for the AF13 PHB. Note that the scheme will preserve ordering of
packets since AF11 and AF13 use a single queue..
In order to provide this service, the provider will have to
implement the AF11 and AF13 PHBs in core network equipment. The
AF11 and AF13 PHBs can be implemented for example, using a single
RIO queue. The provider will also have to provision equipment
within the core of the provider's network to provide the AF11/AF13
service. By provisioning the appropriate RED parameters, for
example, the provider is able to control the priority of AF11
traffic relative to AF13 traffic at each network node. Since there
are no quantitative guarantees, the provider can be quite liberal
in its provisioning strategy and may realize significant
statistical multiplexing gains. Also, the absence of quantitative
guarantees makes it easy to provide this type of service across
multiple DS provider domains. This is because is not necessary to
negotiate, then provision and enforce quantitative guarantees at
multiple boundaries.
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5.2 Leased Line Emulation Service
This is a quantitative service which emulates traditional leased
line service. As such, it promises to deliver customer traffic
with very low latency and very low drop probability, up to a
negotiated rate. Above this rate, traffic is dropped. Such a
service is typically offered between two specific points. It is
suitable for many customer applications. However, due to the high
quality guarantees, it is likely to be priced higher than
alternate services and therefore, to be used only for applications
which really require this type of service. An example of such an
application is IP telephony. A corporate customer might purchase
leased line emulation service between each pair of a number of
corporate network sites.
5.2.1 Service Implementation
In this example, we consider a customer with three geographically
dispersed networks interconnected via a single provider network.
Customer attachment points are represented as A, B and C. At each
attachment point, an SLS describes the leased line service to be
provided to the other two points. The table below represents the
information required in the TCS at attachment point A:
EF-Mark : 100 Kbps : Egress point B : Discard non-conforming
traffic
EF-Mark : 50 Kbps : Egress point C : Discard non-conforming
traffic
Packets submitted for leased line service should be marked with
the DS-field codepoint corresponding to the EF PHB [EF]. From the
ingress point A, to the egress point B, the provider is promising
to carry up to 100 Kbps of traffic. Excess traffic will be
discarded. From ingress point A, to egress point C, the provider
promises to carry 50 Kbps of traffic. Of course, there is some
tolerance required in policing the traffic and thus, there may be
a specification of tolerated jitter or burst size. However, for a
leased line service, the primary traffic profile parameter would
be the sustained traffic rate.
The provider will provision a policer at ingress point A to limit
traffic destined for egress point B, to 100 Kbps. Similarly, a
policer will be configured to limit traffic destined for egress
point C, to 50 Kbps. These policers will require classification
based on the DS-Mark and the destination address in each packet.
In order to provide this service, the provider will have to
implement the EF PHB in core network equipment. The EF PHB can be
implemented using strict priority queuing or alternatively, by
assigning EF marked packets to a heavily weighted queue in a WFQ
scheme. The provider will have to provision equipment within the
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core of the provider's network. For example, routers carrying
traffic between point A and points B and/or C will have to be
provisioned considering the resources committed by the TCS at
point A. This means that a router which is both in the path from A
to B and from A to C, will have to be considered to have committed
150 Kbps of bandwidth as a result of the TCS in place at A. A
router that is only in the path from A to B, will have to be
considered to have committed 100 Kbps as a result of this TCS, and
so on. Of course, routing is subject to change and so, failover
paths may have to be provisioned as well. These may be provisioned
to provide some fraction of the service in the case of failover or
alternatively, the SLS at point A might reflect appropriate
service availability parameters. To enhance the assurances offered
by EF service, providers may employ route pinning mechanisms or
QoS routing mechanisms.
5.3 Quantitative Assured Media Playback Service
This service offers looser assurances than the leased line service
described above, but is still considered a quantitative service.
In particular, it promises to deliver traffic with a high degree
of reliability and with variable but bounded latency, up to a
negotiated rate. Above this rate, traffic is subject to
significant delay or drop. Such a service is typically offered
between a specific set of points. It is suitable for many customer
applications. It would likely be priced lower than a leased line
service, due to the latency variability. However, due to the
latency bound and high degree of delivery, it is likely to be
priced higher than alternate services. This service is
particularly suitable for video or audio playback, in which
considerable bandwidth is required on a continual basis, but the
non-interactive nature of the traffic makes it somewhat delay
tolerant.
5.3.1 Service Implementation
In this example, we again consider a customer with three
geographically dispersed networks interconnected via a single
provider network. The table below represents the information
required in the TCS at attachment point A:
AF11-Mark : 100 Kbps sustained, 100 Kb bursts tolerated at up to
200 Kbps : Egress point B : Excess burst traffic over sustained
rate marked with AF12-mark : Non-conforming traffic marked with
AF13-mark : Max latency = 1 second
AF11-Mark : 50 Kbps sustained, 100 Kb bursts tolerated at up to
100 Kbps : Egress point C : Excess burst traffic over sustained
rate marked with AF12-mark : Non-conforming traffic marked with
AF13-mark : Max latency = 2 seconds
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Packets submitted for the assured playback service should be
marked with the DS-field codepoint corresponding to the AF11 PHB.
From the ingress point A, to the egress point B, the provider is
promising to carry up to 100 Kbps of sustained traffic with bursts
of 100 Kb in size at a peak rate of 200 Kbps. Excess burst traffic
will be marked with the codepoint for AF12 and out of profile
traffic will be carried but with the AF13 codepoint. So long as
these conditions are met, latency will be limited to 1 second.
Note that for this service, the traffic profile is described using
a full set of token bucket parameters. Since the latency bounds
for such a service are less strict than those required for the
leased line service, a certain degree of traffic burstiness can be
tolerated.
The provider must support the AF11, AF12 and AF13 PHBs in core
network routers. These PHBs might be provided, for example, by
assigning AF11, AF12 and AF13 marked traffic to a single RIO queue
with high drop thresholds. The policers at the boundary would
limit competing traffic in line with the TCS, in order to assure
that the latency bounds can be met. In addition, the service
provider will have to provision devices in the core of the
network. The provisioning considerations discussed in the context
of the leased line service apply here as well, however, in
general, the service provider has the liberty of being less
conservative in provisioning and realizing better statistical
gains.
5.4 Superposition of Quantitative and Qualitative Services in the
Same Network
A compelling model would provide both quantitative and qualitative
services in the same differentiated service network(s) as follows.
A number of corporate campus networks would be interconnected by a
differentiated service network providing quantitative services
between the sites. For example, a mesh of leased line services
would enable IP telephony between the sites. A mesh of media
playback service using the AF11 PHB would enable audio/video
playback between the sites. In addition, each corporate site would
be allotted some level of BBE service to arbitrary destinations.
In this model, the differentiated service network is effectively
providing a mesh of quantitative services between fixed locations
(similar to a VPN). This mesh is superimposed on a cloud
supporting BBE service.
6. Provisioning and Configuration
The provision of differentiated services requires careful network
wide provisioning and configuration. Provisioning refers to the
determination and allocation of the resources needed at various
points in the network. Provisioning may dictate the addition or
removal of physical resources at various points (physical
provisioning). Provisioning may also dictate the modification of
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operating parameters within existing physical network equipment to
alter the relative share of the equipment's resources which are
allotted to one or another class of traffic (logical
provisioning). Configuration refers to the distribution of the
appropriate operating parameters to network equipment to realize
the provisioning objectives.
In Secs. 4.6 and 4.7, we briefly discussed provisioning and
configuration requirements both at the network boundaries and in
the network interior. In this section we will focus primarily on
the coordination of provisioning and configuration throughout the
network, such that end-to-end services can be provided reliably.
We will discuss the roles of protocols such as SNMP, CLI, RSVP,
COPS and LDAP in the provisioning process.
6.1 Boundary vs. Interior Provisioning and Configuration
For the sake of brevity, consider the term 'provisioning' to refer
both to provisioning and configuration, except where otherwise
noted. It is helpful to consider provisioning at the network
boundaries, separately from provisioning of the interior. Since
the differentiated service provider is selling a contract (SLA) at
the network boundary, we can consider the boundary provisioning
which supports SLSs, to drive the interior provisioning. The two
are not entirely separable in that each affects the other. For
example, a network operator cannot offer an SLS which cannot be
met by the resources available in the interior of the network. In
general, the overall provisioning process iterates between the
boundaries and the interior. From here on we will refer to
provisioning with respect to the TCS rather than the SLS, since
the TCS is the component of the SLS that defines detailed traffic
handling parameters.
6.1.1 Boundary Provisioning
Boundary provisioning was considered briefly in Sec. 2.6. We
discussed the minimal provisioning that a provider must implement
to enforce a TCS. We also discussed additional configuration which
a provider may use to provide additional (especially per-flow)
services to a customer. The latter is not actually related to the
provisioning of resources within the differentiated services
network, but rather assists the customer by determining which
subsets of the customer's traffic make use of the resources
provisioned within the differentiated services network. As such,
it is out of the scope of this section. Here, we consider only the
minimal provisioning required at the boundary.
At a minimum, the provider must assure that sufficient physical
resources are provisioned at the boundary to meet the requirements
of the TCS. For example, if the sum of the profiles supported at a
particular ingress point would allow 10 Mbps of traffic to be
supported, it is unacceptable to provision a T-1 access link. A T-
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3 however, would be sufficient. Once the physical provisioning is
implemented, it is necessary to apply the appropriate logical
provisioning. This is achieved by configuring policers which limit
the amount of traffic accepted from the T-3 access link, at each
service level and, for double ended TCSs, for the appropriate
egress points. It may also be necessary to set up the amount of
buffering available for the queues used for the service. Similar
provisioning is also appropriate at each egress point if the
aggregate of profiles provisioned to the egress exceeds the
capacity of the output link.
6.1.2 Interior Provisioning
For the purpose of provisioning the interior of the network, it is
desirable to understand or to control the volume of traffic of
each class which traverses each network node. The greater this
understanding, the more efficiently the network can be provisioned
while still meeting the requirements of the TCSs. It is feasible
to understand the volume of traffic traversing each node if this
traffic is admitted according to a TCS which dictates egress point
as well as ingress point. (This case generally applies to
quantitative services and was discussed in the context of the EF
PHB and the leased line service in Sec. 3.2.1). While traffic
volumes cannot be anticipated with 100% accuracy, it is possible
to approximate them quite well, especially with the help of route
pinning mechanisms. It is therefore possible to provision the
network reasonably accurately for traffic submitted for
quantitative services. Although such provisioning may be quite
difficult in a large network, it is nonetheless a tractable
problem. We will refer to this component of the provisioning
problem as quantitative provisioning.
On the other hand, many (if not most) of the services offered by
differentiated service networks will not specify egress points (as
is the case for qualitative services) and will not restrict
submitted traffic to specific egress points, let alone specific
routes. Thus, interior nodes will have to be provisioned without
an a-priori understanding of the volume of traffic submitted for
qualitative services which will arrive at each node. It is
necessary to be able to provision differentiated service networks
to support both quantitative services with specific egress points
as well as qualitative services, which do not have specific egress
points on the same physical resources. To this end, it is
necessary to isolate the impact of qualitative traffic on the
resources reserved for quantitative traffic. This can only be
achieved if the former is treated with lower priority than the
latter. Thus, in general, resources will have to be provisioned
first for quantitative traffic, using quantitative provisioning
mechanisms. Then, qualitative provisioning can be used to allocate
remaining resources to qualitative traffic. Qualitative
provisioning can also be applied to services which offer a
relative quantification of traffic volumes.
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The impact of the two types of traffic will have to be isolated by
ensuring that they do not share PHB codepoints. PHBs used for
quantitative services will always have higher priority access to
resources than those used for qualitative services. As a result,
it is necessary to carefully police traffic submitted for
quantitative PHBs. Failure to do so can result in the starvation
of lower priority traffic. In general it can be expected that only
a small fraction of the resources at each node will be provisioned
for quantitative traffic.
Similarly, a significant fraction of the traffic capacity should
remain for best-efforts service to provide a 'soft' target for
traffic dropping if congestion occurs or it is necessary to
redirect non-best efforts traffic in the event of failure.
6.1.2.1 Quantitative Provisioning of the Interior
As discussed previously, quantitative provisioning is a difficult
but tractable problem. With knowledge of the network routing
topology and the TCSs at the boundaries, it is possible to compute
the resources required at each interior node to carry the
quantitative traffic offered at the edges. Based on the results of
this computation, interior nodes must be provisioned and
configured with sufficient capacity to accommodate the
quantitative traffic which will arrive at the node, while leaving
sufficient capacity remaining to accommodate some amount of
qualitative traffic.
The provisioning mechanism described assumes a top-down approach,
in which the network administrator studies the network topology
and traffic routing and computes the provisioning requirements. An
alternative approach uses signaling to automate the process
[MPLS]. For example, RSVP messages could be launched along the
paths that will be followed by submitted quantitative traffic. If
a TCS calls for 100 Kbps of leased-line service from ingress point
A to egress point B, an RSVP message could be transmitted from
point A towards point B, with a flowspec specifying 100 Kbps. This
message would traverse each node at which resources would have to
be committed. Conventional RSVP routers would install a
reservation. In a differentiated service network, RSVP could be
adapted to provision the resources required per the differentiated
services model. In a network which offers a number of static TCSs,
such RSVP messages could be launched from the TCS ingress point at
the time the TCS is initially instantiated with the effect of
instantiating the appropriate cumulative provisioning in routers
along the various routes. The advantage of this approach is that
it does not require explicit knowledge of the network topology. We
will revisit these two approaches to quantitative provisioning of
the interior in a later section.
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Once the resources required for quantitative traffic at each node
have been determined, provisioning of the node consists of
installing or configuring interfaces of the appropriate capacity
to easily accommodate the quantitative traffic that will traverse
the node. Note that we do not state the precise meaning of 'to
easily accommodate'. A number of factors must be considered when
determining the appropriate capacity, given a certain volume of
predicted quantitative traffic. These include:
1. Margin of error
2. Statistical gain desired
3. Capacity remaining for qualitative (including best efforts)
traffic
The first, margin of error, accommodates mistakes in computation,
effects of transient route changes which are not otherwise
accounted for, effects of traffic clustering as it moves through
the network and so on. The statistical gain desired refers to the
degree to which a provider is willing to gamble that not all
sources of quantitative traffic will be simultaneously active at
the limit dictated by the TCSs at the ingress points (vs. the
penalty the provider would be willing to pay in terms of refunded
charges or lost customers). Finally, the provider must determine
how much capacity will be reserved for qualitative traffic at each
node. Thus, if it is determined that 1 Mbps of quantitative
traffic might traverse a specific node in a specific direction,
the provider might install a 10 Mbps interface in the node, to
serve the corresponding traffic direction. This would leave 9 Mbps
of capacity quite safely for qualitative traffic. In this case,
the provider would be assuming that statistical gains which might
be realized will be used to offset the margin of error which would
compromise the resources available.
In addition to installing or configuring the appropriate capacity
at each interface, it may be desirable to configure policers to
assure that the resources actually consumed by the higher priority
quantitative traffic do not exceed expectations. This is
especially important if the provider is attempting to achieve a
high degree of statistical gain or has not allowed for a
reasonable margin of error. Policers need not be configured at
each interior node, but should probably be configured at certain
key nodes. It may also be necessary to configure the internal
resources of the router (queues and buffers) to deliver the
services required.
6.1.2.2 Qualitative Provisioning of the Interior
As explained previously, it is necessary first to determine the
resources which must be provisioned at each node for quantitative
traffic. Once these have been determined, interfaces must be
installed or provisioned to accommodate the required resources
while leaving sufficient capacity for qualitative traffic. In
order to do so, it is necessary to determine the resources
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required at the node for qualitative traffic. Since qualitative
traffic cannot be assumed to follow specific routes with the same
degree of predictability as quantitative traffic, this
provisioning problem is far more difficult and provisioning
parameters must be estimated based on heuristics, experience and
possibly on real time measurement.
Once physical interfaces have been selected to accommodate the
resources required by the computed quantitative traffic load and
the estimated qualitative traffic load, additional configuration
is required to support qualitative traffic. Such configuration
amounts to the selection of relative weights for queues for
different service levels (in a WFQ scheme), or the selection of
RIO or RED thresholds or alternate logical resource provisioning
parameters. It is assumed that if quantitative traffic is
accommodated via similar queuing mechanisms (as opposed to strict
priority queuing), that the weighting parameters chosen for
quantitative traffic isolate it effectively from the effects of
qualitative traffic. However, the configuration parameters which
differentiate the various qualitative services may not provide
such a degree of isolation among the qualitative services. Thus,
it may be necessary to attempt to estimate the relative traffic
arriving for each qualitative service and to anticipate the
interaction between traffic of different qualitative services. It
may be impossible to both efficiently and conservatively provision
a network for certain combinations of qualitative services. To aid
in the provisioning of a network for qualitative services, it may
be useful to configure policers to control the volume of traffic
arriving at a given node. However, such policing might have to be
restricted to shaping (rather than discarding) in order to avoid
violating TCSs in place at the network boundaries.
6.2. Static vs. Dynamic Provisioning
So far, we have considered static provisioning techniques. Even
the example of RSVP usage for provisioning assumed that the RSVP
messages were launched at the time a TCS was instantiated as
opposed to dynamically. In the case that TCSs are static, static
provisioning is adequate for quantitative traffic. However, since
qualitative traffic offers less predictable patterns, it is likely
to cause traffic volumes at different nodes in the network to
change dynamically, even when the TCS is static. For this reason,
dynamic provisioning techniques are desirable and may assist the
service provider in making better use of network resources. In
addition, dynamic provisioning may enable the service provider to
provision more liberally for quantitative services, realizing
statistical gains. If we consider further, that it may be
desirable to provide dynamically changing TCSs, then the appeal of
dynamic provisioning techniques is even stronger.
Dynamic provisioning may be signaling based, measurement based or
both. For example, a conventional RSVP router supports signaling
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based dynamic provisioning. Hosts signal the router to request
more or less resources and the router adjusts accordingly. The
host may or may not actually submit traffic at the rate at which
it signaled it would, but regardless, the resources are committed
in case it does. Measurement based provisioning would adjust the
resources committed in response to the traffic loads actually
measured at the device. While differentiated services does not
specify any form of signaled or measurement based provisioning,
both may be useful.
6.3 Distributing Configuration Information
The process of physical provisioning is by necessity relatively
static and cannot be automated since it requires installation of
physical equipment. However, logical provisioning and
configuration can and should be automated to the degree possible.
In this section, we look at techniques for distributing
configuration information.
6.3.1 Top Down Distribution of Configuration Information
In the simplest case, TCSs are static and both the boundaries and
interior of the network are provisioned statically by 'pushing'
configuration information down to the appropriate network nodes.
Configuration of boundary nodes requires primarily the pushing of
policing information to enforce the TCSs in place. (Additional
fine grain information may be pushed to provide traffic separation
services on behalf of the customer, but these are not addressed in
this context). Configuration information for boundary nodes is
determined at the time the TCS is negotiated. At this time, the
nodes are configured by the provider. The network administrator
may use one of several protocols to do so, including for example
SNMP or CLI.
In order to accommodate the traffic submitted by the provisioning
of new TCSs, it is necessary to provision the interior of the
network. As discussed previously, it is possible to compute the
resources required for quantitative traffic. Assuming that
sufficient physical capacity has been provisioned, configuration
amounts to logically provisioning sufficient capacity at each
interior interface and to configuring policers for the
quantitative traffic at various interior nodes. In addition,
qualitative provisioning requires the configuration of queues, WFQ
weights and/or RIO parameters at various interior nodes, and may
also include the configuration of some number of policers. In the
case, of static, top down configuration, interior configuration
information is also pushed down via a configuration protocol such
as SNMP or CLI.
The difficulty of such top down provisioning is that it requires
the network administrator to coordinate the provisioning of each
network node, at boundaries as well as in the interior, such that
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the network is provisioned end-to-end in a consistent manner and
is able to efficiently deliver the services promised by the TCSs.
In order to assist the network administrator in this task, it is
useful to consider a database which holds both current topology
information as well as the current TCSs instantiated at the
network boundaries. This information is stored in a format
dictated by a standard schema as suggested in [Ellesson].
Of course, the database is ideally maintained in a way which is
logically centralized (for ease of programming and modifying) but
is physically distributed (for the sake of robustness and fault
tolerance). Policy servers may be used to extract information from
the database and to convert it to configuration information which
is pushed down to individual nodes. In this scenario, policy
servers would likely use a directory access protocol such as LDAP
to retrieve information from the directory and would use a
configuration protocol such as SNMP or CLI to push the
configuration information down to the network nodes. Note that in
this example, the policy servers and the directory schemas are in
effect fulfilling the role of bandwidth broker [ROTZY]. In
particular, the policy servers use an awareness of the network
topology to provision interior nodes such that certain end-to-end
QoS routes can be constructed and assurances implied by the TCSs
at the boundaries can be delivered.
6.3.2 Distribution of Configuration Information via Signaling
An alternate mechanism of distributing configuration information
is via signaling messages transmitted between boundary nodes of
the same differentiated service domain (intra-domain signaling).
It is also interesting to consider inter-domain signaling, but
this will be addressed separately. An example of such signaling
was described previously, in the usage of a modified form of RSVP.
Such signaling is particularly useful for the purpose of
installing configuration information for quantitative services
which affect specific paths and is somewhat less useful (though
not useless) for the purpose of configuring qualitative services.
It is likely that such a signaling approach would be used in
conjunction with top down provisioning. For example, the directory
schema might dictate the amount of resources to be available for
high priority quantitative services at each node. These limits
might be pushed down to individual nodes a-priori. Signaling from
the network boundaries, at TCS instantiation time, would then be
used to claim resources from the pool of quantitative resources
available at each node. Alternatively, nodes might consult policy
servers as the signaling resource requests arrive at each node.
The latter model is similar to the use of per- flow RSVP signaling
and PEP/PDP policy usage in traditional RSVP networks. Qualitative
configuration information would still be pushed in a top down
manner. The advantage of the latter model is that policy servers
would be dynamically updated with information regarding the
current usage of network resources. In this model, it is likely
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that a variant of COPS would be used to communicate between
network nodes and the policy servers. Note that COPS may be used
for distribution of top down configuration information as well,
though it is not specifically designed for this purpose.
One of the advantages of configuration via signaling, is that it
facilitates the support of dynamic TCSs. TCSs could be dynamically
renegotiated using inter-domain signaling. Such renegotiation
would require dynamically modifying the provisioning within the
affected domain, a process which requires some automated signaling
protocol such as an aggregated form of RSVP signaling between
boundary nodes in a provider's domain. This protocol would in
effect, represent a distributed bandwidth broker [ROTZY] for the
domain.
6.3.3 Modification of Configuration Information Based on Real-Time
Measurement
A third mechanism for the configuration of interior nodes would be
based on measurement of current traffic loads at key network
nodes. Measurement based configuration is less necessary for
quantitative provisioning, since quantitative traffic patterns are
relatively predictable. However, it can significantly enhance the
efficiency with which qualitative provisioning can be achieved.
For example, network nodes may feed policy servers with current
qualitative traffic load measurements. In response, bandwidth
brokers and policy servers might recompute the relative weights
for different service queues in a WFQ node and push the new
configuration information to the routers. It is likely that
measurement based configuration for qualitative services would be
used in conjunction with signaling based configuration for
quantitative services.
7. Inter-Domain Considerations and End-to-End Services
So far we have considered differentiated service primarily in the
context of a single DS domain providing service to a single
customer. The ultimate customers of the differentiated service
network are hosts and end users residing on peripheral stub
networks. In general, these are interconnected by multiple domains
and require service which spans these domains. Therefore, it is
important to consider the interaction of services provided by a
concatenation of differentiated service domains and the peripheral
stub networks, rather than the service provided by a single
domain. The interactions of the services and the network
concatenation present a serious challenge to providers seeking to
provision the services scientifically. Whether algorithms or
heuristics can be developed to cover the full spectrum of service
combinations is an open question, but by analogy with QoS Routing
it is very likely that some of the problems are not computable.
In this section, we discuss inter-domain issues related to TCSs,
provisioning and service and PHB mapping.
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7.1 TCSs
Each service provider is expected to negotiate bilateral
agreements at each boundary node at which it connects to an
adjacent provider's network. Such The technical aspects of these
agreements that relate to delivering differentiated services are
captured in the form of two TCSs, one specifying the services
provided to provider A's traffic by provider B and the other
specifying the services provided to provider B's traffic by
provider A. Note that provider A serves as a provider to provider
B with respect to traffic flowing from provider B to provider A.
On the other hand provider A is a customer of provider B with
respect to traffic flowing from provider A to provider B. The two
TCSs can be considered separately.
In general, the TCSs needed by a provider at any boundary will be
dictated by TCSs negotiated at other boundaries. For example,
assume that provider A offers leased line service to a customer
with an ingress point in provider A's domain, but an egress point
in provider B's domain. In this case, it is necessary that the TCS
between provider A and provider B be sufficient to accommodate the
assurance made by provider A to its leased line service customer.
Provider A may serve a number of customers with leased line
services terminating at various boundary points in provider B's
network. Thus, the TCS between provider A and provider B must
represent the aggregate requirements of the TCSs of all of
provider A's customers.
7.2 Inter-Domain Provisioning
The inter-domain provisioning problem is not unlike the intra-
domain provisioning problem. The provider would generally begin by
evaluating the TCSs it has negotiated with its customers, and then
computing the impact of each of these TCSs on the TCSs it has
negotiated with its providers. For quantitative services, the
provider can compute the quantitative requirements of TCSs at each
of its provider's boundary nodes, as described above in the
context of the leased line service. For qualitative services, the
process of determining the requirements from its providers is
fuzzier, since the volume of qualitative traffic expected to be
carried through any boundary is less deterministic.
In the simplest case, provisioning is based on static TCSs. In
this case, provisioning is an iterative process in which providers
negotiate TCSs with each of their customers, then apply the
appropriate internal provisioning techniques to meet these
requirements. In the process of internal provisioning, a provider
might determine that a particular TCS cannot be met due to
internal resource constraints. The provider would then either have
to add internal resources or renegotiate one or more customer
TCSs. Although the process may be somewhat iterative, it is
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relatively static in that changes in boundary TCSs and internal
provisioning occur relatively infrequently (on the order of hours,
days or months) and require human intervention.
Internal provisioning to meet the requirements of TCSs relies on
provisioning techniques described previously. As TCSs are
negotiated, the provider must check that the existing internal
provisioning is sufficient to meet the requirements of the new
TCS, or must alter the internal provisioning. Recall that internal
provisioning might be pushed in a top down manner, from a domain's
logically centralized point of administration, or alternatively
might be distributed from the boundaries via signaling. In the
former case, some form of a bandwidth broker would be directly
consulted or notified regarding changes in TCSs negotiated at the
domain boundaries. In the case that signaling is used,
provisioning messages (such as described previously) would be
launched from the boundary at which the new TCS is negotiated.
These would claim a share of existing provisioned resources, or
would notify the bandwidth broker in the case that additional
resources are required.
A more sophisticated model would allow TCSs to be renegotiated
dynamically. In this case, the process would be automatic, and
would not require human intervention. Each domain would in effect,
represent a bandwidth broker, via one protocol or another. A
specific inter-domain protocol might be used to communicate
between centralized bandwidth broker agents, or alternatively, an
inter- domain variant of RSVP might be used. In the latter case,
there is no direct interaction with a bandwidth broker per-se.
However, the collection of network nodes, policy servers and
directory behave collectively as a bandwidth broker which
communicates using RSVP. In either case, TCS renegotiations would
be triggered by load measurements at boundary nodes. These could
be in the form of changes in actual measured traffic volume, or
alternatively, based on explicit fine grain RSVP resource requests
from hosts at the periphery. Domains would approve renegotiations
based both on resource constraints as well as predetermined policy
constraints.
7.3 Service, PHB and Codepoint Mapping
In order to provide end-to-end service to customers, it must be
possible to extend services across multiple domains. Several
complexities may arise at inter-domain boundaries, as follows:
1. The services provided by a certain domain may not be compatible
with the services provided by a neighbour domain.
2. The services provided by a certain domain may be compatible
with those provided by the neighbour domain, but the PHB used
to obtain the service might be different.
3. The PHB might be the same, but the codepoint used to request
the PHB might be different.
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4. The PHB and codepoint are the same but differences in
provisioning and charging models results in different services.
Resolution of these complexities requires determination of the
compatible services and negotiation of the PHB codepoints which
will be used to request the services. This process is greatly
simplified by the provision of a set of universal services using
universally recognized codepoints. The leased line service and the
recommended EF codepoint is likely to be one such example.
Generally, extension of quantitative services across multiple
domains will require more uniformity in the nature of the services
provided. Qualitative services on the other hand, may be extended
end-to-end by a concatenation of services which vary from domain
to domain. For example, one domain may base a qualitative service
on a WFQ scheme with RED while another may use priority queuing
with RIO. Since the assurances provided by qualitative services
tend to be looser, it is possible that a meaningful service can be
provided end-to-end by concatenating these two service types.
7.4 Host-Domain Boundaries
In certain cases, a host may be directly attached to a
differentiated service domain. This is likely both in the case of
campus networks that provide differentiated services within the
network or in the case of dial-up users connecting to a
differentiated service provider. In these cases, the host can be
considered the customer of the differentiated service network.
Legacy hosts are unlikely to mark their own packets for the
appropriate DS-field and are also unlikely to shape or police
their traffic. In the case of legacy hosts, the differentiated
service provider will have to provide these services on behalf of
the customer. In the case of campus networks, some network wide
policy would likely be used to configure these services in the DS
boundary devices. In the case of dial-up hosts, marking, shaping
and resources provided would likely be negotiated at the time the
customer signs up with the provider.
Newer hosts may be capable both of marking and of traffic shaping.
In this case, the overall per-host resource constraints are still
likely to be somewhat static. However, the manner in which the
host shares these resources among its various traffic flows is
determined by the host. Of course, the provider will have to
configure policers to assure that the host does not seize more
than its share of resources in the differentiated service network.
8. Deployment Scenarios
A number of scenarios can be envisaged for the junction of a non-
DS Domain and a DS Domain, and hence for deployment scenarios:
1. A service provider runs a DS Domain which offers Differentiated
Services to a customer who has a network which has no DS
capability - the junction occurs at the ingress to the service
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provider's network. The service provider would provide
classification, marking and shaping of traffic as a value-added
service using information provided by the customer.
2. A service provider runs a DS Domain for a customer who has a
network which has mostly no DS capability except that the
customer's first hop or demarcation router acts as a
degenerate, one node DS Domain. The only (boundary) node in
this domain performs classification, marking and shaping,
whilst the provider's equipment just has to police the incoming
aggregate traffic.
3. The customer and provider both have fully capable DS Domains.
Hosts are embedded in the customer's DS Domain - the junction
between the non-DS and DS Domains is logically at the boundary
between the Operating System and the application.
Scenarios 1 and 2 provide simple initial deployment mechanisms for
DS as they do not require general modification of hosts. The
advantage of Scenario 2 over Scenario 1 is that the customer can
use, and keep private, local administrative knowledge to improve
the classification of packets. In Scenario 1 this information
would have to be made available in the service provider's domain
to achieve the same granularity of classification requiring that
the customer have greater trust in the provider.
Scenario 3 requires modification of hosts. Direct interaction
between applications and the DS Ingress node would therefore be
possible giving scope for sophisticated application of the DS
capabilities; even without such interaction, extremely fine-
grained classification of traffic packets would be possible in the
operating system kernel. Authentication of the application/host
and authorization to use DS services requires particular attention
in this case, although care has to be taken to avoid denial of
service attacks in all cases (see Sec. 11 and [DSARCH] for further
discussion of security).
A customer might also deploy a network of DS capable routers
before some or any of the associated hosts were DS capable.
Classification, marking and shaping would be provided by the
`first hop' router which the packet encounters on the first hop
after leaving the host; the core of the customer's network is
fully DS capable and the packets are forwarded in accordance with
their DSCP to another host in the same DS Domain or on to a
provider's domain.
It might be possible to utilize non-DS capable routers in the
interior of a DS Domain without compromising the QoS delivered
provided:
- The non-DS capable routers forward unchanged TOS byte all
packets marked with the values of DSCP used in the DS Domain.
- The non-DS capable routers forward these packets as if they
were best efforts traffic
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- The non-DS capable routers are used only at points which rarely
or never experience congestion.
9. Inter-operability with RSVP/Integrated Services
In this section, we discuss alternatives for inter-operability
between differentiated services and RSVP/Integrated services.
9.1 RSVP/Integrated Services Over Differentiated Services
This scenario is discussed in detail in [E2EQOS]. It assumes a
model in which peripheral stub networks are RSVP and Intserv
aware. These are interconnected by differentiated service
networks. In this model, the scalability of differentiated service
networks helps to extend the reach of RSVP/Integrated service
(Intserv)networks. Intervening differentiated service networks
appear as a single RSVP hop to the RSVP/Intserv networks. Hosts
attached to the peripheral RSVP/Intserv networks signal to each
other for per-flow resource requests across the differentiated
service networks. Standard RSVP/Intserv processing is applied
within the RSVP/Intserv peripheral networks. RSVP signaling
messages are carried transparently through the differentiated
service networks. Devices at the boundaries between the
RSVP/Intserv networks and the differentiated service networks
process the RSVP messages and provide admission control based on
the availability of appropriate resources within the
differentiated service network.
This model is predicated on the availability of services within
the differentiated service network which can extend the reach of
intserv type services. For example, the leased line service can
extend the intserv guaranteed service across a differentiated
service network. Multiple guaranteed service micro-flows which
exist in peripheral networks are aggregated into the EF behaviour
aggregate at the boundary of the diffserv network. When an RSVP
request for guaranteed service arrives at the boundary of a
differentiated service network, RSVP style admission control is
applied based on the amount of resources requested in the intserv
flowspec and the availability of differentiated services at the
corresponding service level (per the TCS). If admission control
succeeds, the originating host (or its agent) marks traffic on the
signaled microflow, for the appropriate differentiated service
level.
The RSVP/Intserv over differentiated service model is especially
suitable for providing quantitative end-to-end services. The use
of differentiated services eliminates the scalability concerns of
RSVP/Intserv networks. The use of RSVP signaling provides
admission control to the differentiated service network, based on
resource availability and policy decisions. It also greatly
simplifies the configuration of differentiated service
classifiers, policers and other traffic conditioning components.
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Variations on this theme would enable some number of nodes within
the differentiated service networks to process the per-flow RSVP
messages passing through. These could be used to aid in dynamic
provisioning without necessarily requiring any per-flow state or
processing within the differentiated service network. In yet
another model, the transition of per-flow RSVP messages through
the differentiated service network might trigger aggregated RSVP
signaling between differentiated service domain boundaries, for
the purpose of renegotiating TCSs and adjusting provisioning
dynamically [GBH97, CLASSY].
9.2 Parallel Operation
Another alternative for the interoperation of differentiated
service and RSVP/Intserv networks is simple parallel operation. In
this mode, each node within the differentiated service network may
also be an RSVP capable node. Some strategy would have to be
selected for determining which packets are handled using RSVP and
which are handled using differentiated services. For example,
those that classify to an RSVP installed filter might be handled
using RSVP, while those not classifying to specific RSVP filters
would be handled according to the DS-field using differentiated
service mechanisms. Such a model is likely to be deployed in
smaller networks (since the RSVP/Intserv component is less suited
for large networks). In particular, the stub networks cited in
[E2EQOS] would likely provide differentiated services for those
qualitative applications which do not signal, while providing
RSVP/Intserv services for those quantitative applications which do
signal.
10. Multicast Services
The basic concept of Differentiated Services appears to offer an
excellent fit with a multicast service insofar as traffic may be
forwarded from an ingress to several egresses. Unfortunately, as
we shall see, provisioning a multicast service is extremely
difficult.
Because the Differentiated Services Architecture deals only with
unidirectional flows, a 'multicast' service in a DS network will
in fact offer a point-to-multipoint unidirectional service. Each
source of traffic that wishes to send to the multicast group using
this service needs a separate SLS which applies at the ingress
point where the traffic enters the network.
The network resources that must be provisioned for a multicast
service will be affected by the mechanisms used by the routers to
provide the service. Depending on the capabilities of the routers
and the multicast routing protocol employed, sub-optimal
replication of a packet may result in multiple copies travelling
over the same link.
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If receivers can be added dynamically to a multicast group whilst
a flow is in progress, the complexity of provisioning grows
considerably: The amount of network resources that will be
consumed by multicast traffic originating from a particular
upstream network may be difficult to forecast in advance.
Consequently, it may not be possible to offer quantitative
services where dynamic addition of receivers adds to the paths
through the network already used by the flow.
All multicast receivers must also be capable of handling the
existing or proposed traffic on the multicast tree. This is an
extension of the receiver control problem discussed in Sec. 4.4.1
where it is clearly not desirable for a single inadequate receiver
to limit the traffic on a complete tree. It is therefore
essential that a multicast service specify a minimum receiver
capacity _ where the service passes from one domain to another the
TCS on the receiving domain must offer at least this capacity.
Note that application level multicast does not normally fall into
the multicast service category because it is normally realised as
a number of independent unicasts each of which is delivered by a
unicast service.
10.1 Codepoints and PHBs for Multicast Services
To achieve resource isolation of multicast traffic from unicast
traffic, it may be necessary to use separate codepoints and
separate instances of a PHB or different PHBs for the multicast
and unicast services. If the multicast traffic is not adequately
isolated, dynamic addition of new members of the multicast group
can adversely affect existing unicast traffic.
Because a multicast service traffic flow can exit from a domain to
several peer domains, care must be taken to use a codepoint and
PHB that is compatible with the peering SLSs at the egress points.
This may be a more stringent requirement than for a unicast
service where a flow need only be compatible with a single egress
point SLS.
10.2 Provisioning Multicast Services
The scope of a multicast service would normally be either case 1
(any egress point) or case 3 (a pre-defined set of egress points)
of Sec. 4.4.
For a quantitative service the scope will, in general, need to be
case 3. The service can be provisioned in a similar way to
corresponding unicast services with the same volume of traffic
along each of the paths from ingress to egress, but taking into
account that all paths will be used simultaneously and allowing
for multiple copies of traffic if necessary. If the multicast
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routing protocol used can generate different multicast trees
depending on the order in which members join the group,
provisioning may not be possible. Solving this problem may
require pinning of the multicast tree branch points; the solution
of this problem is outside the scope of this framework.
For a qualitative service, provisioning is essentially the same as
the unicast case, but statistical multiplexing gains are likely to
be less because all paths may be used at once.
The traffic conditioning mechanisms for multicast services are not
significantly different from those for the unicast services but
multiple shapers may be required where traffic exits from several
interfaces on a single router or multiple replicas exit from one
interface.
An additional problem arises when a service is actually used as
part of a multipoint-to-multipoint service. The traffic patterns
resulting from this usage and the required provisioning cannot be
easily generalised from the point-to-multipoint case, with the
result that it is difficult to determine how much extra capacity
should be provisioned when a link is a common path for traffic
from several sources.
11. Security and Tunneling Considerations
The security and tunneling implications for the actual data
transport of the services of the Differentiated Services
Architecture have been extensively discussed in [DSARCH] and
[DSHEAD] to which the reader is referred.
Additional security considerations arise from the services
overlaid on the data transport:
1. The services maybe the subject of differential charging.
Accordingly, the service users have to be authenticated and
authorized, and the accounting data needed must be secured.
2. The mechanisms used to create and distribute the policy and
resource allocations must be secured.
3. Statistical data needed to audit service delivery must be
secured.
The mechanisms used to provide this security are outside the scope
of this framework, but are under consideration by the AAA working
group.
The use of tunnels in general and IPsec tunnels in particular
impedes the work of MF Classifiers by concealing the fields used
by L4 and higher layer classifiers. Thus traffic conditioners
within the area where IPsec encryption is used will need to rely
only on IP header fields, including the DS Field (BA Classifiers
will work normally). If more sophisticated MF classification is
required it will have to take place before the tunnel ingress and
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the application of IPsec encryption. If IPsec encryption is used
end-to-end, then Differentiated Services may require host marking
Similarly, there is a constraint on quantitative services in
general because IPsec hides the final destination address, so that
it may be difficult to police quantitative services when IPsec is
used because the traffic conditioner cannot determine the egress
address easily.
If a tunnel carries multiple flows with different traffic types,
they may be marked with different DS codepoints so that they are
subjected to appropriate behaviors in the network interior. This
may be considered to be a security breach as it allows traffic
patterns to become visible. If just one codepoint is used for all
traffic it should be selected carefully to be appropriate for all
the traffic in the tunnel.
12. Acknowledgements
The authors would like to acknowledge the helpful comments and
suggestions of the following individuals: Kathleen Nichols, David
Black, Konstantinos Dovrolis, Shivkumar Kalyana, Wu-chang Feng,
Marty Borden, and Ronald Bonica.
13. References
[2BIT] K. Nichols, V. Jacobson, and L. Zhang, "A Two-bit
Differentiated Services Architecture for the Internet", Internet
Draft
[CLARK] D. Clark and J. Wroclawski, "An Approach to Service
Allocation in the Internet", Internet Draft
[CLASSY] S. Berson and S. Vincent, "Aggregation of Internet
Integrated Services State", Internet Draft, November 1997.
[COPS] J. Boyle, R. Cohen, D. Durham, S. Herzog, R. Rajan, and A.
Sastry, "COPS (Common Open Policy Service) Protocol", March 1998.
[DSARCH] D. Black, S. Blake, M. Carlson, E. Davies, Z. Wang, and
W. Weiss, "An Architecture for Differentiated Services", Internet
Draft, May 1998.
[DSHEAD] K. Nichols and S. Blake, "Definition of the
Differentiated Services Field (DS Byte) in the IPv4 and IPv6
Headers", Internet Draft, May 1998.
[AF] J.Heinanen, _Assured Forwarding PHB Group_Internet Draft,
August 1998.
[EF] V.Jacobson, _Expedited Forwarding Per Hop Behavior_,
Internet Draft, August 1998.
Bernet, et al Expires: September 1999 35
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[Ellesson] E. Ellesson and S. Blake, "A Proposal for the Format
and Semantics of the TOS Byte and Traffic Class Byte in IPv4 and
IPv6", Internet Draft, November 1997.
[E2EQOS] Y. Bernet, R. Yavatkar, P. Ford, F. Baker, L. Zhang, K.
Nichols and M. Speer, "A Framework for the Use of RSVP with Diff-
serv Networks", Internet Draft, November 1998.
[DiffEdge] Y. Bernet, D. Durham and F. Reichmeyer, _Requirements
of Diff-serv Boundary Routers_, Internet Draft, November 1998.
[ROTZY] F. Reichmeyer, L. Ong, A. Terzis, L. Zhang, and
R. Yavatkar, _A Two-Tier Resource Management Model for
Differentiated Services Networks_, Internet Draft, November 1998
[MPLS] B. Thomas, N. Feldman, P. Doolan, L. Andersson and
A. Fredette, "Label Distribution Protocol Specification", Internet
Draft, January 1999.
[GBH97] R. Guerin, S. Blake, and S. Herzog, "Aggregating RSVP-
based QoS Requests", Internet Draft, November 1997.
[IntServ] R. Braden, D. Clark, and S. Shenker, "Integrated
Services in the Internet Architecture: An Overview", Internet RFC
1633, July 1994.
[RSVP] B. Braden et. al., "Resource ReSerVation Protocol (RSVP) --
Version 1 Functional Specification", Internet RFC 2205, September
1997.
[RFC791] Information Sciences Institute, "Internet Protocol",
Internet RFC 791, September 1981.
[RFC1349] P. Almquist, "Type of Service in the Internet Protocol
Suite", Internet RFC 1349, July 1992.
14. Author's Addresses
Bernet, Yoram
Microsoft
One Microsoft Way
Redmond, WA 98052
Phone: +1 (425) 936-9568
Email: yoramb@microsoft.com
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Binder, James
3Com Corp.
5400 Bayfront Plaza
Santa Clara, CA 95052
Phone: +1 (408) 326-6051
Email: james_binder@3com.com
Blake, Steven
Torrent Networking Technologies
3000 Aerial Center, Suite 140
Morrisville, NC 27560
Phone: +1-919-468-8466 x232
Fax: +1-919-468-0174
Email: slblake@torrentnet.com
Carlson, Mark
RedCape Software Inc.
2990 Center Green Court South
Boulder, CO 80301
Phone: +1 (303) 448-0048 x115
Email: mac@redcape.com
Carpenter, Brian E
IBM United Kingdom Laboratories
MP185
Hursley Park
Winchester
Hampshire SO21 2JN
UK
Phone: +44 1962 816833
Email: brian@hursley.ibm.com
Davies, Elwyn
Nortel Networks
London Road
Harlow, Essex CM17 9NA, UA
Phone: +44-1279-405498
Email: elwynd@nortelnetworks.com
Ohlman, Borje
Ericsson Radio
Dialoggatan 1 (Kungens Kurva)
S-126 25 Stockholm
Sweden
Phone: +46-8-719 3187
Email: Borje.Ohlman@ericsson.com
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Srinivasan Keshav
4107B Uspon Hall
Cornell University
Ithaca, NY 14853
Phone: +607-255-5395
Email: skeshav@cs.cornell.edu
Dinesh Verma IBM T. J. Watson Research Center
P.O. Box 704
Yorktown Heights, NY 10598
Phone: +1 (914) 784-7466
Email: dverma@watson.ibm.com
Zheng Wang
Bell Labs Lucent Tech
101 Crawfords Corner Road
Holmdel, NJ 07733
Email: zhwang@bell-labs.com
Walter Weiss
Lucent Technologies
300 Baker Avenue, Suite 100 Concord, MA 01742-2168
Email: wweiss@lucent.com
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